1. Field of the Invention
The present invention relates to reading data in a disk drive and compensating for an offset between a read element and a write element of a head.
2. Background Information
Hard disk drives contain a plurality of magnetic heads that are coupled to rotating disks. The heads write and read information by magnetizing and sensing the magnetic fields of the disk surfaces. There have been developed magnetic heads that have a write element for magnetizing the disks and a separate read element for sensing the magnetic fields of the disks. The read element is typically constructed from a magneto-resistive material. The magneto-resistive material has a resistance that varies with the magnetic fields of the disk. Heads with magneto-resistive read elements are commonly referred to as magneto-resistive (MR) heads.
Each head is attached to a flexure arm to create a subassembly commonly referred to as a head gimbal assembly (xe2x80x9cHGAxe2x80x9d). The HGA""s are suspended from an actuator arm. The actuator arm has a voice coil motor that can move the heads across the surfaces of the disks.
Information is typically stored in radial tracks that extend across the surface of each disk. Each track is typically divided up into a number of segments. The voice coil motor and actuator arm can move the heads to different tracks of the disks.
FIG. 1 shows a typical sector of a disk. The sector contains a servo address mark (SAM) that provides a sync for a SERVO field. The SERVO field contains servo bits that are used to center the head on the track. A DATA field follows the servo field. The data field typically contains a preamble that is used to phase lock the circuits of the disk drive with the information on the disk.
The head is typically connected to a pre-amplifier circuit that has a read gate and a separate write gate. Enabling the write gate allows information to be written onto the disk through the write element of the head. Enabling the read gate allows information to be read from the disk through the read element.
As shown in FIG. 1, the write gate WG is typically enabled in conjunction with the trailing edge of a SECTOR signal. The preamble and data are then written onto the disk. There are typically multiple DATA sectors, wherein the process of generating a SECTOR signal and writing data is repeated. The SECTOR signals are typically generated at predetermined time intervals after the detection of the SAM signal. When reading the data the read gate RG is enabled at approximately the trailing edge of the sector signal.
The write element is typically offset from the read element of an MR head. Consequently, some of the preamble is actually written prematurely so that the read element misses a portion of the preamble during a read routine. The preamble is used to phase lock the circuits of the disk drive to allow for proper reading of the data in the subsequent DATA sector(s). Without a sufficient amount of preamble to read, the disk drive may not acquire proper phase lock. A lack of phase lock may cause errors in reading the data. This problem can be alleviated by increasing the length of the preamble so that there is enough preamble data to allow phase lock. Unfortunately, stretching the preamble reduces the storage capacity of the disk drive.
A hard disk drive that includes a head coupled to a disk. The head includes a read element offset from a write element. The drive further includes electrical circuits that enable the read element as a function of the offset.